Simulation modelling

Simulation involves the modelling of discrete events, e.g. transaction arrival rates, against a given hardware configuration. This type of modelling can be very accurate in sizing new applications or predicting the effects of changes on existing applications, but can also be very time-consuming and therefore costly.

When simulating transaction arrival rates, have a number of staff enter a series of transactions from prepared scripts, or use software to input the same scripted transactions with a random arrival rate. Either of these approaches takes time and effort to prepare and run. However, it can be cost-justified for organizations with very large services and systems where the major cost and the associated performance implications assume great importance.

4.3.5.8 Application sizing

Application sizing has a finite lifespan. It is initiated at the design stage for a new service, or when there is a major change to an existing service, and is completed when the application is accepted into the live operationalenvironment. Sizing activities should include all areas of technology related to the applications, and not just the applications themselves. This should include the infrastructure, environment and data, and will often use modelling and trending techniques.

The primary objective of application sizing is to estimate the resource requirements to support a proposed change to an existing service or the implementation of a new service, to ensure that it meets its required service levels. To achieve this, application sizing has to be an integral part of the Service Lifecycle.

During the initial requirements and design, the required service levels must be specified in an SLR. This enables the Service Design and development to employ the pertinent technologies and products to achieve a design that meets the desired levels of service. It is much easier and less expensive to achieve the required service levels if Service Design considers the required service levels at the very beginning of the Service Lifecycle, rather than at some later stage.

Other considerations in application sizing are the resilience aspects that it may be necessary to build into the design of new services. Capacity Management is able to provide advice and guidance to the Availability Management process on the resources required to provide the required level of performance and resilience.

The sizing of the application should be refined as the design and development process progresses. The use of modelling can be used within the application sizing process.

The SLRs of the planned application developments should not be considered in isolation. The resources to be utilized by the application are likely to be shared with other services, and potential threats to existing SLA targets must be recognized and managed.

When purchasing software packages from external suppliers, it is just as important to understand the resource requirements needed to support the service. Often it can be difficult to obtain this information from the suppliers and it may vary, depending on throughput. Therefore, it is beneficial to identify similar customers of the product and to gain an understanding of the resource implications from them. It may be pertinent to benchmark, evaluate or trial the product prior to purchase.

Quality must be built in.

Some aspects of service quality can be improved after implementation (additional hardware can be added to improve performance, for example). Others – particularly aspects such as reliability and maintainability of applications software – rely on quality being ‘built in’, since to attempt to add it at a later stage is, in effect, redesign and redevelopment, normally at a much higher cost than the original development. Even in the hardware example quoted above, it is likely to cost more to add additional capacity after service implementation rather than as part of the original project.